The trend in electronics today is to systems of ever increasing component packing density. Increased component density permits designer to achieve greater speed and complexity of system performance within system size constraints by virtue of the large number of circuit elements they can use to perform circuit functions. Additionally, increased component density enables circuit manufacturer to lower production costs owing to the economies they can realize using integrated circuit technology.
The desire for increased component density has given rise to the so-called very large scale integrated (VLSI) circuit. In such circuits, designers pack large numbers of components into individual integrated circuit chips. Subsequently, the designers gang these chips on a single substrate to form larger circuits and functional blocks for the system. To accommodate such high density circuits, designers have developed multilayer ceramic (MLC) substrates which permit the many terminals of the chips to be interconnected in a space efficient manner.
MLC substrates are well known and have been described in such articles as "A Fabrication Technique for Multilayer Ceramic Modules" by H. D. Kaiser et al appearing in Solid State Technology, May 1972, pp. 35-40. As Kaiser et al explain in their article, ceramic green sheet, i.e. ceramic powder held together in sheet form by temporary organic binders are metallized using conventional screen printing. The metallized sheets are stacked, laminated and thereafter fired, i.e. cured, to form a monolithic package. This structure provides a three-dimensional wiring system for the chip interconnections in what was formally waste or inaccessible space in the ceramic substrate. The use of this "waste space" results in the creation of a high-density electronic package suitable for handling the high-density integrated circuit chips. An example of a sophisticated embodiment of a semiconductor module including a multilayer ceramic substrate is described in U.S. Pat. No. 4,245,273 assigned to the assignee of this application.
As noted, MLC makers form wiring patterns on the multilayer ceramic by screening a metallic paste consisting of metal powder in an organic binder onto the ceramic. The metal used is selected to be susceptible of being powdered, slurried, screened and fired to form the desired chip interconnect metallization pattern on the ceramic. Most typically, a refractory metal such as molybdenum which is capable of withstanding the substrate curing temperatures is selected.
Following formation of the cured MLC substrate, designers add additional metal layers to the initial molybdenum to obtain desirable bondability characteristics, i.e. a metallization to which the integrated circuit chip may be readily joined. As is well known, molybdenum alone is for practical purposes impossible to bond to. Accordingly, a more bondable metarial such as nickel is used to overlay the molybdenum. To further improve the bond characteristics, a layer of gold may be added over the nickel.
Unfortunately, because of random and non-uniform shrinkage during curing, makers of MLC substrates have been limited in the process techniques that are available to them for applying the subsequent metallization layers. Particularly, because the ceramic shrinks randomly and non-uniformly when cured, MLC makers find that each substrate molybdenum metallization pattern is caused to randomly shrink and distort uniquely. This unique distortion destroys the original size and regularity of patterns when first screened. As a result, conventional photolithographic techniques using a standard master mask have therefore been impossible for MLC makers to use as a method for applying subsequent metal to each of the cured MLC substrates.
To overcome the difficulties associated with random and non-uniform shrinkage, MLC makers have used so-called electroless plating processes. In accordance with electroless plating, the cured MLC having the initial molybdenum metallization on it is immersed in a bath. The bath is prepared to include metal ions which when reacted, selectively plate at the substrate, i.e. at the molybdenum metallization pattern. To effect the reaction, a reducing agent is also included in the bath. The reducing agent supplies the necessary electrons to the metal ions to render them neutral so they may "plate out" of the solution. To induce metal plating only at the substrate metallization, the concentration of metal ions and reducing agent is held low enough so that a catalyst will be required to effect the metal reduction reaction. The catalyst is selectively added to the substrate prior to the substrates immersing in the bath so that the catalyst is present only at the substrate metallization pattern. When the substrate is immersed in the bath, the metal ions are reacted only at the substrate metallization. Since the reaction occurs only at the substrate metallization, the plating is self-aligning and substantially independent of the pattern plated. Accordingly, the difficulties arising from substrate distortion and irregularity due to curing are avoided.
There are, however, difficulties associated with electroless plating. For example, electroless plating is sensitive to process parameter such as temperature, metal ion and reducing agent concentration, application of the catalyst, etc., as well as contaminants on the substrate. As a result, the electroless process can be difficult to control. Where the process is not properly controlled, MLC makers find plating may occur at other than the substrate metallization. Additionally, the layer plated may not adhere to the metallization process or be of non-uniform character. All such results render the plated MLC commercially unacceptable.